As used in the present application, the term ‘liquid active principle’ constitutes the generic definition covering all the previously mentioned embodiments of the active principle.
Needleless injection devices comprising a reservoir of liquid active principle already exist in tube form and have been the subject of patents. For example, the patent application FR 2 853 837 may be cited, which relates to a needleless injection device comprising an injection nozzle and a tube intended to receive a liquid active principle to be injected, said tube being fixed to said nozzle with the help of connecting means.
The person skilled in the art traditionally seeks to minimize or even eliminate the risks of damaging, on the one hand, the tube containing the liquid active principle and, on the other hand, the downstream stopper in the specific ‘double stopper’ configuration described, in particular, in the patent application WO 01/58512. Said configuration is characterized by the presence of a column of liquid delimited, on the one hand, by the side wall of the tube and, on the other hand, by an upstream stopper and a downstream stopper, between which the liquid active principle is housed. Under the effect of the gas generation, said column moves in the tube until the downstream stopper comes into contact with the base of a receptacle located in the nozzle so as to free the injection ducts in order to expel said active principle. However, when the downstream stopper hits the base of said receptacle, it creates a shockwave that will propagate through to the tube and the intensity of which is a maximum at the end of said tube which is in contact with the nozzle. In a ‘double stopper’ configuration, the downstream stopper and the glass tube are the two most stressed elements during the operation of the needleless injection device. The latter must consequently be shaped in such a way that the speed of impact of the downstream stopper in the receptacle is less than the limit speed acceptable for the mechanical strength of the components of said injection device. In addition, it is known that the time separating the opening of the injection ducts and the start of the output of the jet from the injection ducts must be longer than the attenuation time of the pressure oscillations generated by the opening of the injection ducts.
In the technical literature of the field in question it is traditionally mentioned that it is necessary to apply, to the liquid active principle to be injected, on the one hand a high pressure between 200 and 500 bar in order to pierce the skin, and on the other hand a constant or decreasing pressure through to the end of the injection in order to ensure the transfer of the liquid active principle, the final values generally being between 40 and 250 bar.
However, there is a major stake in reducing the operating pressures so as to construct strong and economical needleless injection devices, more particularly for single-use devices or the consumable parts, such as the injection nozzle, of reusable devices while guaranteeing good reproducibility of injections.